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The disclosure of this patent application includes an Appendix which is a copy of a paper to be published shortly after this application is filed.
1. Field of the Invention
This invention relates generally to cathodic protection systems for corrosion protection of metal objects which are buried in soil. More particularly, the invention relates to an apparatus and method for continuously monitoring the adequacy of cathodic protection being applied at each of a multiplicity of monitoring stations installed next to and spaced along buried objects, such as pipelines and underground pipe-type electrical power transmission cables, and for automatically controlling, or providing data for manual control of, each individual one of a multiplicity of rectifiers spaced along the same buried object based upon data gathered from multiple neighboring monitoring stations.
2. Description of the Related Art
Buried metal structures, such as pipes, pipe-type cables and storage tanks, are subjected to natural, electrochemical corrosion processes in their underground environment. Although non-conductive coatings are typically applied to such buried objects, the coatings develop flaws or pinholes, known as holidays, that allow the metal object to be directly exposed to the soil electrolyte. This exposure to the soil electrolyte not only permits local corrosion at each holiday, but additionally permits stray electrical currents from other buried objects and above ground sources to be conducted through the soil electrolyte to the buried object and thereby cause further electrochemical corrosion.
Cathodic protection systems have long been applied to buried objects to counteract or at least mitigate the electrochemical corrosion process. Impressed current systems utilize one or more rectifiers to apply a voltage through suitable conductors across the buried object and a buried anode. Typically, these cathodic protection rectifiers are located at spaced intervals along the buried object. Sacrificial anode cathodic protection systems are also used and they are typically also spaced along the buried object. They are powered only by the xe2x80x9cbatteryxe2x80x9d which is inherently formed by the dissimilar metals of the anode and buried object connected together by a conductor and immersed in the soil electrolyte.
The technology of cathodic protection recognizes the existence of potentials at the interface between a soil electrolyte and a buried metal. A first relevant potential is the free corrosion potential, also referred to as the oxidation potential, which is the potential at a corrosion interface associated with the oxidation-reduction reactions. Over a period of time, the flow of current causes polarization at the interface and the buildup of an opposing potential as a characteristic of the polarization. Consequently, after polarization, the component corrosion potential and polarization exhibit a resultant potential which is the polarized potential. If the current is halted, the polarization is gradually reduced over time, and the resulting potential is referred to as the depolarized potential. As discussed below, the depolarized potential may not be the same value as the initial free corrosion potential.
Various criteria have been suggested and some have been widely adopted as industry standards for defining a degree of cathodic protection which is considered adequate and are based upon these potentials. One standard (NACE) for typical steel buried objects is the existence of a potential on the buried object which is 850 millivolts more negative than the potential of the standard copper/copper sulfate (CSE) reference electrode placed on top of or buried in the soil. Another standard (NACE) is the existence of a polarized potential which is 100 millivolts more negative than the depolarized potential of the buried metal object.
One prior art method for monitoring this potential difference, to determine whether a particular criterion is met involves installing a standard reference electrode at one or more locations along the buried object together with a conductor connected to the buried object. A technician periodically visits the location and measures the voltage between the standard reference electrode and the buried object using a voltmeter. A responsible person then examines the collected data looking for inadequate voltages and variations from earlier data and, when control is inadequate or excessive, adjusts the output(s) of the appropriate rectifier(s).
Underground stray electrical currents may arise from electrical power installations such as underground power distribution lines or electrically powered mass transit systems. Such stray currents introduce additional complexities and difficulties because they are typically not uniformly distributed along the buried object and may occur more intensely at particular localities and because the magnitude and direction of stray ground currents typically vary with time. DC stray currents contribute to corrosion by causing rapid metal loss at buried object locations serving as current discharge points and by introducing an additional IR-drop in the soil which is a source of error in grade level potential measurements.
The most commonly used potential survey method used to ascertain the existence of the stray current problem for a buried object has been the close interval survey where the pipe-to-soil potential is recorded as a function of distance along the buried object of interest. Any potentials which departed significantly from the potentials at other regions or which deviated significantly from prior surveys, could be identified as possible stray current pickup or discharge points. This has been done by having pipeline maintenance personnel travel to the buried object and make the measurements described above. Single location, time-dependent surveys were also performed by the maintenance staff by monitoring the potential at a chosen location as a function of time. Any significant changes in the recorded potentials could indicate possible intermittent pickup or discharge of stray current.
Advances in technology permitted maintenance staff to carry data loggers, which download data into a computer memory, instead of manually recording measurements from voltmeters. Further advances have permitted some remote detection of the relevant voltage at an individual location using a data communication system to avoid travel to the location.
The technology has been further advanced by the introduction of coupons. A coupon is a metal electrode, buried in the soil, and having essentially the same metallurgical composition as the metal of the buried metal object which is being protected. Apparatus and methods for using coupons are illustrated in U.S. Pat. Nos. 5,814,982 and 6,107,811. These two patents are herein incorporated by reference because the subject matter described in those patents may be used and are preferred for use in connection with the subject matter of the invention disclosed herein.
With coupon technology, a bare coupon is buried near the buried metal object with the coupon normally electrically shorted to the buried object through an electrical lead wire so that the coupon is subjected to the same cathodic protection as the buried object. To permit an accurate potential measurement locally from the grade level, a plastic tube is installed around the coupon to minimize the IR drop in the polarized coupon potential measurement. The coupon-pipe circuit can be interrupted thereby interrupting the cathodic protection of the coupon as well as any stray currents to the coupon and permitting an accurate measurement of the coupon-to-soil off-potential without interrupting the protection to the buried pipe itself. The prior art measures this off potential, EOFF, using a standard reference electrode. Typically, the off potential is measured a short time interval, such as 100 milliseconds, after the connection to the cathodic protection system is interrupted. This interval permits the system to settle and transients to die out but is short enough so that no significant depolarization of the coupon has occurred.
The prior art has also used a form of feedback control for controlling a cathodic protection rectifier. In such a system, an analog feedback control system monitors a pipe-to-soil potential with the cathodic protection turned off (some did it with the cathodic protection remaining on) and this measurement was used in a conventional feedback control loop to control the voltage applied by a single rectifier associated with the measurement location.
A deficiency in the prior art exists because stray currents affect the buried object differently at different locations along its length as a result of stray currents arising from other discrete buried objects, such as neighboring, cathodically protected pipelines. Therefore, there is often a wide variation in stray current characteristics at different locations along the length of the buried object. In addition to these substantial spatial variations, stray currents are often dynamic in nature because they vary with time on an hourly, daily and seasonal basis. Consequently, there is a need for a cathodic protection monitoring and control system which can respond to both the spatial and the temporal variations in cathodic protection needs. There is, therefore, a need for a system which can gather and utilize voltage measurement and other data inputs from multiple monitoring sites spaced along the buried object and is able to provide both spatial and temporal responsiveness.
Furthermore, there is a need for apparatus and methods which can utilize the inputs from multiple monitoring sites to individually adjust one or more rectifiers in response to data obtained from multiple monitoring sites along the buried object in order to optimize system resources and minimize cost while providing adequate protection to each segment of the buried object.
In the past, potential measurements which were made to determine the adequacy of the cathodic protection have often been made with reference to the standard reference electrode, such as a copper/copper sulfate reference electrode. There is, however, a need for an improved measuring method and for an improved reference electrode for potential measurements which will be more durable, more accurate and will account for variations in soil conditions, such as seasonal changes in soil moisture, to allow measurements which more accurately reflect the actual protection applied to the buried metal object despite changes in soil conditions.
This invention is directed to an automated data collection and monitoring system, and additionally a control system, for a cathodic protection system having a multiplicity of impressed current cathodic protection circuits, such as rectifiers, electrically connected to the buried object. A multiplicity of test stations are also installed at spaced intervals along the object. Each test station has at least one buried reference electrode, a polarized coupon switchable into and out of electrical connection with the object and having a metallurgical composition similar to the object, and a voltage detection circuit in electrical connection to, and for measuring the potential between, the polarized coupon and the reference electrode. A data communication telemetering system for transmitting potentials detected by the voltage detection circuit connects each of the test stations to a remotely located central control-processing unit which acquires data, as detected at each test station, representing at least the potential, EOFF, of the polarized coupon when that coupon is disconnected from the buried object and preferably also the potential, EON, of that coupon when it is connected and the current ION flowing between the polarized coupon and the buried object. Preferably the reference electrode also has a metallurgical composition substantially the same as the object. Preferably the central control processing unit analyzes the data and determines the adequacy, inadequacy and excessiveness of the cathodic protection of the buried object in the vicinity of each test station. The central computer may also be connected through the telemetering system to one or more of the cathodic protection system rectifiers and be programmed with a control algorithm for controllably adjusting the voltage or current of the cathodic protection system which is applied to effect protection. Some of the principles of the invention are also applicable to discrete buried objects, such as a buried storage tank, which utilizes only a single test station and a single cathodic protection circuit.